RE: STP Question - Port Fast

Hello Faisal,
 
By itself, portfast does not turn off spanning-tree. Portfast only gets
very aggressive with the forward delay time, essentially skipping the
listening and learning states. For example, if you take a cross-over cable
and link two portfast ports on the same switch, one of them will go into
blocking state.
 
However there is more to the story.
 
Spanning tree protocol operates as a CPU process on the switch. IF the
switch is otherwise overwhelmed, it is possible that putting the forward
delay time to zero would result in a loop (it is at 15 seconds for a
reason!).
 
More, there are two OPTIONAL features that alter the STP behavior of
portfast ports:
One is bpdu guard, the other is bpdu filter. But these are optional,
additional features.
 
Hope that helps,
 
Bob Sinclair CCIE 10427 CCSI 30427
www.bobsinclair.net
 
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Faisal Ilyas
Sent: Friday, December 24, 2010 2:54 PM
To: Cisco certification
Subject: STP Question - Port Fast
 
Salam,

As per Port Fast enable on cisco switches, Does the Spanning tree loop
detection is still in operation, i have read in a BCMSN book CH9 that the
port moves into the blocking state if a loop is ever detected on the port
fast port.

--
Regards,
M. Faisal.
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